The Fredenstein F660 Limiting Amplifier is a valve (tube) based compressor and limiter with the same very short signal path as the famous Fairchild 660/670.
It uses the exact type of tubes, the 6386 LGP, as the Fairchild. Fortunately, new production tubes are available today, since original tubes are hardly sold anymore. The four tubes form a single stage push-pull Class A amplifier and the whole signal path consists only of a T-type input attenuator, input transformer, Class A amplifier and finally an output transformer.
At this point the similarities end. The F660 uses a DSP to control the operating points of the tubes, guarantying perfect working conditions all the time. The side chain amplifier is designed with semi-conductor components since it only generates the control voltage for the tubes. The final result is a compressor with the compression characteristics of the Fairchild, but with an extended frequency response, lower noise and lower distortion. The perfect buss compressor is for today's high resolution audio.
Frequency Response: 20Hz 20Khz +- 0.5dB
Distortion: < 0.1% (no compression)
Signal-to-Noise Ratio: >85db at 0db gain
Gain Range: -14dB to +11dB minimum
Threshold: -24dBu to + 16.5dBu
Mains Voltage 110V to 120V in the 115V range
Power Consumption: < 100W
or 220V to 240V in the 230V range